Typically, the production of sponge iron in a vertical shaft, moving bed reactor involves two principal steps; namely, reduction of the ore in a reduction zone through which is passed a suitable hot reducing gas largely composed of carbon monoxide and hydrogen at temperatures in the range of 700.degree. C. to 1000.degree. C., preferably 750.degree. C. to 950.degree. C., and cooling of the reduced sponge iron in a cooling zone through which is passed a gaseous coolant at a temperature below about 200.degree. C., preferably below 100.degree. C. Systems of this general type are disclosed in U.S. Pat. Nos. 3,765,872, 3,816,102, and 4,216,011 wherein a vertical reactor is used having a reduction zone in the upper portion thereof and a cooling zone in the lower portion thereof. The ore to be treated is charged to the top of the reactor and caused to flow downwardly through the reduction zone wherein it is reduced by heated reducing gas, after which the reduced ore flows into and downwardly through the cooling zone to be cooled and carburized by contact with a stream of suitable cooling gas. The cooled sponge iron is then discharged through the bottom of the reactor. Typically, both the reducing gas and cooling gas are recirculated, optionally in closed loops, to which streams of fresh (i.e. "make-up") reducing gas are added and from which streams of spent gas are removed.
In various known direct reduction processes, the reducing gas required for reduction of the iron ore is generated in a catalytic reforming unit by conversion of natural gas in accordance with the following reactions: EQU CH.sub.4 +H.sub.2 O.fwdarw.CO+3H.sub.2 ( 1) EQU CH.sub.4 +CO.sub.2 .fwdarw.2CO+2H.sub.2 ( 2)
In the reforming reactions the natural gas comprised mainly of methane is converted to hydrogen and carbon monoxide in the presence of an oxidizing agent of either water or carbon dioxide. As a result, the reformed gas is substantially composed of hydrogen and carbon monoxide. In recent times, due to the ever decreasing availability and increasing cost of natural gas it has become extremely important and therefore desirable to develop a direct reduction process in which the required quantity of natural gas is minimized.
The reducing gas being fed to the reduction zone of the reactor is typically at an elevated temperature and is caused to contact the downwardly moving iron ore to reduce the iron oxides therein according to the following basic reactions: EQU 3Fe.sub.2 O.sub.3 +H.sub.2 /CO.fwdarw.2Fe.sub.3 O.sub.4 +H.sub.2 O/CO.sub.2 ( 3) EQU Fe.sub.3 O.sub.4 +H.sub.2 /CO.fwdarw.3FeO+H.sub.2 O/CO.sub.2 ( 4) EQU FeO+H.sub.2 /CO.fwdarw.Fe+H.sub.2 O/CO.sub.2 ( 5)
The spent reducing gas leaving the reactor is cooled to remove water produced by the reduction of the iron ore with hydrogen after which the cooled and de-watered effluent gas is recycled to the reduction zone of the reactor. The recirculation, or recycling, of the effluent gas can be accomplished in various ways; for example by recycling the gas directly back into the reactor or by recycling the gas first through the reformer and heater or by recycling the gas through the heater only. In each case, however, fresh reducing gas is added to the recycled effluent gas prior to injection into the reactor. Since the amount of carbon dioxide generated in the process by the reduction reactions occurring in the reactor is considerable, a portion of the spent gas must be vented or purged from the system to maintain a proper overall carbon balance within the reduction system.
As described above, the fresh reducing gas which is typically generated by the catalytic conversion of methane present in the natural gas fed to the process constitutes a net carbon feed to the process due to the carbon content of the natural gas. In order to operate the reduction process under steady state conditions and in a continuous manner, it is necessary to remove carbon from the system in an amount essentially equivalent to the net amount of carbon being added. Carbon can leave the system in combined form with the sponge iron in the form of ferric carbide or in gaseous form as CO, CO.sub.2 and CH.sub.4 being vented from the spent gas recycle loop.
A proper balance between the amount of carbon entering and leaving the process is necessary to avoid excessive carbon deposition on the sponge iron burden moving through the reactor. Although carbon can be effectively eliminated from the process by venting at least a portion of the effluent gas from the reactor, hydrogen which is also present in the gas effluent and is particularly effective as a reductant is also lost from the process.
Therefore, a real need exists for a direct gaseous reduction process in which carbon can be eliminated from the process without attendant loss of valuable reducing components such as hydrogen and methane. A need also exists for a reduction process which provides a greater amount of flexibility and control of the level of metallization and carburization of the sponge iron product. Such flexibility of operation is desirably achieved simultaneous with optimization of the overall energy consumption of the direct reduction process.